1. Field of the Invention
The present invention relates to a technology for correcting color misalignment in an image forming apparatus.
2. Description of the Related Art
Occurrence of color misalignment is a critical problem in color image forming apparatuses. For this purpose, typical color image forming apparatuses have a function of detecting and reducing color misalignment. To implement such a function, in one approach, toner patterns of different colors are formed on a transfer belt, those toner patterns are detected with a photosensor, amounts of color misalignment for various causes are calculated based on the result of detection of the tonner patterns, and a feedback control is performed based on the calculated amounts of color misalignment. Examples of the causes include main-scanning-direction misregistration, sub-scanning-direction misregistration, main-scanning-direction magnification error, and skew. A feedback correction for compensating the calculated misalignment amounts is then performed to reduce the misalignment.
A color image forming apparatus performs the feedback correction at various occasions to constantly limit each misalignment amount below a predetermined value. Such feedback correction is performed, for example, when the image forming apparatus is turned on, when the image forming apparatus undergoes an environmental change such as a temperature change, and when a print count of the image forming apparatus reaches a predetermined number.
Examples of the method for correcting the color-to-color misalignment will be explained below. The main-scanning-direction misregistration and the sub-scanning-direction misregistration can be corrected by adjusting write-start timing of a laser beam on a photosensitive drum.
The main-scanning-direction magnification error can be electrically corrected by adjusting a pixel clock.
Skew of a laser beam that performs scanning exposure can be corrected mechanically, or by using an image processing technique. The method of mechanically correcting the skew uses an adjusting mechanism that is used to adjust a position of a mirror inside a laser-beam write unit to correct the skew. However, to implement this method automatically, an actuator such as a mirror-displacing motor is required to move the mirror, which means additional cost. This method is further disadvantageous in making it difficult to configure the laser-beam write unit compact.
The image processing technique for correcting the skew of a laser beam is as follows. A portion of image data is stored in a line memory that has a capacity to store therein one line of image data in the main-scanning direction. Pixels belonging to the one line of the image data in the line memory are then divided into a plurality of pixel blocks. When reading (outputting) the image data in each of the pixel blocks in the line memory, the order of reading the image data in each of the pixel blocks is changed so that the image data is shifted in a direction opposite to a skew direction. Accordingly, color-to-color skew can be corrected. Because this method requires only one additional line memory of a size corresponding to a desired correction area, this method is advantageous in being implementable with a relatively small additional cost as compared with that of the mechanical correcting method. This correcting method based on the image processing technique is effective not only for skew correction but also for reducing the degree of distortion resulting from the property of a lens in the laser-beam write unit or the like.
However, the method based on the image processing technique is disadvantageous in that because relation between neighboring pixels on a shift position changes, a color density can be locally increased or decreased. This can result in banding noise extending in the sub-scanning direction on an output image (for example, an image printed on a printing paper). In particular, such local color density increase or decrease frequently occurs on an image that is processed by using a digital halftoning method such as dithering, and produces banding noise extending in the sub-scanning direction.
Japanese Patent No. 3715349 discloses a conventional technique for correcting skew of an image and reducing banding noise that can result from the skew correction. In the conventional technique, it is determined whether a pixel of interest is at a shift position. If the pixel of interest is at the shift position, and when a neighboring pixel in the main-scanning direction of the pixel of interest has changed and a pixel pattern of pixels in the vicinity of the pixel of interest matches with a predetermined pattern, density correction is performed for the pixel of interest. This density correction is performed according to a set of the matched pattern and the position where the neighboring pixel changes.
Although the conventional technology disclosed in Japanese Patent No. 3715349 teaches to perform the density correction of the pixel of interest, it does not teach how to determine the amount of the density correction. Accordingly, the conventional technology can reduce banding noise by a certain degree but not sufficiently. In particular, in a case of an image having undergone digital halftoning such as dithering, a toner area coverage is likely to be increased or decreased by the shifting of image. Hence, in some cases, banding noise is reduced at certain gray scales while banding noise is stressed rather than reduced at other certain gray scales.